Primary recovery by natural drainage of liquid hydrocarbon deposits may be somewhat scanty, even in the presence of an active aquifer or a free-gas cap, so it is often necessary to use an assisted recovery method, for example by injecting a flushing fluid into the formation.
Water treated to be compatible with the reservoir rock, gases not miscible with hydrocarbons, gases miscible with hydrocarbons, microemulsions, or fluids based on polymers with high viscosity in situ may be injected.
All these processes, well known in the prior art, have the goal of flushing the reservoir rock containing the hydrocarbon as efficiently as possible. This flushing consists of driving the hydrocarbon toward the producing wells.
However, to different degrees depending on their sophistication, these processes all encounter the problem of economic feasibility. Because of the extra cost represented by the injection operations and the products injected, which are often not recoverable, profitability of such a deposit may be low or even zero. Thus it is crucial not to consume a large quantity of these expensive products.
If the displacement agent is polymer-based, the polymers are additives used principally as products that increase the viscosity of the basic fluid, namely water. They can also act on the water in the deposits.
These polymers have the property of adsorbing onto the rock matrix. This adsorption is manifested by retention of product on the rock matrix and hence decreases the polymer concentration in the displacement agent. Moreover, adsorption favors retention of the agent by a trapping phenomenon in the rock matrix. Thus the ratio between the mobility of the displacement agent and that of the hydrocarbon, which depends directly on the concentration, is favorable only in a small area around the injection zone. To remedy this major drawback, large concentrations of polymers may be injected, but this represents a high consumption of injected product and the cost of the operation becomes prohibitive.
U.S. Pat. No. 4,627,494 teaches a process for injecting a chemical into an oil reservoir to limit losses of surfactants used in secondary recovery. However this process relates only to products based on lignosulfonates.
U.S. Pat. No. 3,523,581 describes a process for injecting a first and then a second fluid before injecting the displacement fluid. The second fluid contains a product that can contain polyphosphates in particular, but these are intended only for pre-desorption of the first fluid in the reservoir rock. This method is particularly suitable for reservoirs with nonuniform permeability.